1. Field of the Invention
The present invention relates to a method of measuring the refractive index of a dielectric film and a refractive index measuring apparatus therefor, and more particularly to a method of measuring the refractive index of a dielectric thin film during the formation thereof in a thin film formation apparatus such as a vacuum deposition apparatus and a sputtering apparatus, and a refractive index measuring apparatus therefor.
2. Discussion of Background
When a thin film is formed in a vacuum thin film formation apparatus such as a vacuum deposition apparatus and a sputtering apparatus, the refractive index of a thin film formed is not necessarily the same as the refractive index of bulk material of the same composition as thin film, and usually varies depending upon the deposition conditions and other conditions in the deposition atmosphere during the formation thereof.
In particular, in the case of a reactive vacuum deposition or reactive sputtering, the above-mentioned variations in the refractive index are evident. Therefore, the variation of the refractive index of a thin film formed is inevitable even if there are slight changes in the film deposition conditions. For this reason, it is most desirable to monitor and control the deposition conditions during the formation of a thin film. Such monitoring and controlling of the film deposition conditions are extremely important particularly when optical thin films such as an anti-reflection film and a dichroic mirror are fabricated.
Furthermore, conventionally, it is impossible to measure optical constants such as the thickness of a thin film formed by vacuum deposition on the spot during the formation of the thin film, by using an optical film thickness meter, until a maximum interference value of the thin film is obtained after starting the deposition of the thin film.
Moreover, conventionally, no technique has been known for measuring the refractive index of a thin film with any thickness on the spot during the formation of the thin film. In the formation of an optical dielectric thin film, the deposited dielectric thin film exhibits a maximum value of interference for every one fourth of the wavelength (.lambda.) of a light for measurement of the thickness, that is, .lambda./4. Thus, the thickness of an optical dielectric thin film is conventionally controlled during the formation thereof by utilizing this phenomenon, with a unit of .lambda./4. In addition, it has been reported in Japanese Laid-Open Patent Application 63-28862 that the thickness of such a thin film is controlled as desired by a certain sampling and operation.
However, in any of the above-mentioned measurement methods, the film thickness that can be measured is the optical film thickness (nd), so that it is impossible to measure optical constants of an extremely thin film, particularly on the spot during the formation thereof.